add support for http proxy

[YanKawaYu]

I have the same problem in this issue.
#24 (comment)
I found that apns2 doesn't support http2 proxy. In order to fix the problem, I made some improvements to the following code.

[deltapath-eric]

Hi, seem this changes only available in cert client and the token client is kind of missing?

[coveralls]

Coverage decreased (-8.3%) to 83.818% when pulling 8890cd1 on zhaoxy2850:master into 060d44b on sideshow:master.

[coveralls]

Coverage decreased (-8.3%) to 83.818% when pulling 8890cd1 on zhaoxy2850:master into 060d44b on sideshow:master.

[coveralls]

Coverage decreased (-8.3%) to 83.818% when pulling 8890cd1 on zhaoxy2850:master into 060d44b on sideshow:master.

[coveralls]

Coverage decreased (-8.3%) to 83.818% when pulling 8890cd1 on zhaoxy2850:master into 060d44b on sideshow:master.

[coveralls]

Coverage decreased (-8.3%) to 83.818% when pulling 8890cd1 on zhaoxy2850:master into 060d44b on sideshow:master.

[coveralls]

Coverage decreased (-1.9%) to 90.182% when pulling 37c5474 on zhaoxy2850:master into 060d44b on sideshow:master.

[emiksk]

Will this PR be merged?
I wish this feature is implemented to the library.

[emiksk]

Hi, @sideshow

I want to add this feature in this library.
What do I should do to merge this pull request or add this feature?

[sideshow]

@zhaoxy2850 @emiksk

I agree that there is definitely a need for apns2 to support proxy servers but I am hesitant to merge this as it is because of the following;

2. Its adds two more methods NewProxyClient and NewProxyTokenClient so now we have 4 client constructors. I think a better way is to allow either setting a default Transport/Httpclient or allowing you to pass a custom one into the constructor as i am sure there will be more customisations that others want to do to the transport/httpclient and this will avoid us having to add more methods each time.

3. This uses http2.ConfigureTransport which takes a http1 connection and upgrades it to http2 whereas the other methods used a pure http2 connection. I would rather use the http2 transport directly however the http2 transport doesn't yet support http2 proxying.

For now I have added an example of how to use the proxy code you have in this PR with master as it is today; Its a little bit more code to get set up but it does work and means you can use with a proxy. Please try this and let me know how you get on. Thanks

[YanKawaYu]

Thanks. This can be an alternative way until http2 transport support http2 proxying.

[emiksk]

@sideshow
Thank you so much for your explanation and support.
I understand your opinion. I'll follow the example!